This invention relates to a process for purifying raw water, containing dissolved organic molecules of high molecular weight and silicious contaminants, to render it suitable for use as boiler feed.
In purifying raw water, such as river or lake water, special consideration must be given to the removal of silica in its various forms. Usually the silica is dissolved to some extent and is present in the water as silicic acids. When the untreated raw water is introduced into a boiler, the silicic acids tend to be converted to silica, which forms a solid deposit on the boiler surfaces. This deposit detrimentally interferes with heat transfer within the boiler.
In modern high pressure boilers, which operate at elevated water temperatures, silica and its related compounds (collectively termed ` silicious contaminants`) pose a further problem. Several silicious contaminants are volatile at the temperatures at which high pressure boilers operate and can be carried in the evolving steam, to be deposited in solid form on equipment and pipes through which the steam passes.
It is a well-established feature of boiler operating art that silicious contaminants in boiler feed water may be conveniently removed by contacting the water with a strongly basic anion-attracting exchange resin. The silicious contaminants are adsorbed by the resin.
Before the raw water is contacted with resin, such water is usually treated to remove most of the minerals ("softening") and insoluble organic materials ("clarifying"). This pre-treatment is done by contacting the raw water with certain chemicals, commonly one or more of: lime, alum, carbon, sodium aluminate, and clay. The treatment is done before the water contacts the resin, as the minerals and organic substances themselves tend to be adsorbed onto the resin with the consequence that the resin will rapidly foul and will need regeneration. Regeneration usually involves contacting the resin with a base, such as sodium hydroxide.
A further step in the pre-treatment is contacting the raw water with chlorine to kill obnoxious bacteria.
The combination of traditional pre-treatment and contact with resin has been effective in purifying most types of water used as boiler feed. However, a problem arises when the raw water, besides containing silicious contaminants, also contains significant quantities of large dissolved organic molecules, which is the case when the water originates from regions where drainage areas are swampy. These organic molecules are thought to be humic acid or analogues thereof. In any event, the organic molecules are substantially unaffected by the conventional pre-treatment process; instead, they are adsorbed by the resin and foul it. When the resin is regenerated, these organic molecules are only partly removed by the first regeneration and less completely with subsequent regenerations, with the resin rapidly being rendered useless.
A probable reason for the organic molecules' affinity for the resin is that such resins are manufactured with a high pore volume at the surface to encourage ion exchange; it is believed that this porous structure offers sites where the large organic molecules can adhere to the resin.
Two known methods have been heretofore available for removing the offending water-soluble organic molecules. The first method involves contacting the raw water with adsorbant activated charcoal. However, this method is impractical for large flows. The second method involves contacting the raw water with a weakly-basic anion attracting resin. This resin extracts substantially all organic molecules from the water, after which silicious contaminants may be removed in the usual way with a strongly basic resin. The guard resin may be periodically regenerated, but its use is characterized by: high capital cost, frequent regeneration, and frequent testing to monitor the effectiveness of the guard resin.